A normal human spine is segmented with seven cervical, twelve thoracic and five lumbar segments. The lumbar portion of the spine resides on the sacrum, which is attached to the pelvis. The hips and leg bones support the pelvis. The bony vertebral bodies of the spine are separated by intervertebral discs, which reside sandwiched between the vertebral bodies and operate as joints, allowing known degrees of flexion, extension, lateral bending and axial rotation.
The intervertebral disc primarily serves as a mechanical cushion between adjacent vertebral bodies, and permits controlled motions within vertebral segments of the axial skeleton. The disc is a multi-element system, having three basic components: the nucleus pulposus (“nucleus”), the anulus fibrosus (“anulus”) and two vertebral end plates. The end plates are made of thin cartilage overlying a thin layer of hard, cortical bone that attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The plates thereby operate to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae. The, anulus of the disc forms the disc perimeter, and is a tough, outer fibrous ring that binds adjacent vertebrae together. The fiber layers of the anulus include fifteen to twenty overlapping plies, which are inserted into the superior and inferior vertebral bodies at roughly a 40-degree angle in both directions. This causes bi-directional torsional resistance, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction.
It is common practice to remove a spinal disc in cases of spinal disc deterioration, disease or spinal injury. The discs sometimes become diseased or damaged such that the intervertebral separation is reduced. Such events cause the height of the disc nucleus to decrease, which in turn causes the anulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur. Such disruption to the natural intervertebral separation produces pain, which can be alleviated by removal of the disc and maintenance of the natural separation distance. In cases of chronic back pain resulting from a degenerated or herniated disc, removal of the disc becomes medically necessary.
In some cases, the damaged disc may be replaced with a disc prosthesis intended to duplicate the function of the natural spinal disc. In other cases, it is desired to fuse the adjacent vertebrae together after removal of the disc, sometimes referred to as “intervertebral fusion” or “interbody fusion.” In this process, spondylodesis or spondylosyndesis is used to join two or more vertebrae to eliminate pain caused by abnormal motion, degradation, fractures or deformities of the vertebrae.
Spinal plates have become one common approach to attaching one adjacent vertebra to another. A spinal plate generally includes an elongated plate of a metal such as titanium or stainless steel. The plate includes a plurality of apertures positioned to allow a surgeon to attach the plate across at least two vertebras with screws. The combination of the plate and screws serve to hold the adjacent vertebra together while the intervertebral fusion occurs.
Biomaterials have been used as implants in the field of spine, orthopedics and dentistry including trauma, fracture repair, reconstructive surgery and alveolar ridge reconstruction for over a century. Although metal implants, such as titanium, have been the predominant implants of choice for these types of load-bearing applications, additional ceramics, non-resorbable polymeric and bioresorbable materials have been employed within the last twenty-five years due to their biocompatibility and physical properties.
Polyetheretherketone (PEEK) is a biomaterial often used in medical implants. For example, PEEK can be molded into preselected shapes that possess desirable load-bearing properties. PEEK is a thermoplastic with excellent mechanical properties, including a Young's modulus of about 3.6 GPa and a tensile strength of about 100 MPa. PEEK is semi-crystalline, melts at about 340 degree C., and is resistant to thermal degradation. Such thermoplastic materials, however, are not bioactive, osteoproductive, or osteoconductive.
Biodegradeable graft fixation anchors and screws are made of Biphasic calcium phosphate poly(L-lactide-co-D,L-lactide). Biodegradable polymeric materials such as polylactide and polyglycolide have been used in orthopedic applications for decades. These components are designed to be degraded in the body.
Therefore, there is a need for a series of orthopedic implants, which combine a biocompatible and or biodegradable material or polymer such as, but not limited to, titanium or PEEK or any other material suitable for implantation within an animal in vivo combined with collagen. The collagen should be applied to the implant or the implant should include geometry that allows the implant to be inserted without scraping away the coating. In addition, the collagen coating could provide a lattice for bone in-growth into a portion of the implant to integrate the implant into the bone of the patient or replace the implant with bone as it is absorbed.